1. Field of the Invention
The present invention relates to a color transcoding process and to the corresponding transcoder.
2. Discussion of Background
The invention makes it possible to connect on input equipment with an output equipment. The input equipment incorporates a page memory, whose content is able to define a mosaic-type image formed from characters each defined by a shape, a character color, a background color and various other attributes, the character and background colors being taken from a group of N colors. The output equipment incorporates a means for the display of an image of the mosaic type with the aid of characters also having a shape, a character color and a background color, the character and background colors being taken from a group of M colors, M being smaller than N.
The present invention has a very wide application field. It in particular covers videography which, as is known, is a telecommunications process making it possible to supply to a user alphanumeric or graphic messages on a display screen. In its transmitted variant, this process is often called "teletext" and in its interactive variant is often called "videotex". The invention can also apply to the field of computers or microcomputers, as well as to that of printers, together with various display devices such as flat-faced screens.
The problem which the present invention proposes to solve is that of incompatibility between equipment working with a different number of colors. For example, this is the case when it is wished to display an eight color videography image or picture on a two-color flat-faced screen, or when it is wished to couple a high definition microcomputer using 64 colors with an 8 color printer and so on.
FIGS. 1 and 2 illustrate the position occupied by the transcoder according to the invention in known installations having two incompatible equipments. In FIG. 1, transcoder TR is located between an input equipment EQE and an output equipment EQS. FIG. 2 shows how said same transcoder is inserted in a videography chain having a central processing unit UCT, a page memory MP, a display unit UV and a television receiver RT. The transcoder is then inserted between page memory MP and display unit UV and it makes it possible to control an output equipment EQS.
The invention is applicable in the case where the images to be processed are images of a mosaic type. It is known that such images are formed from characters, each character being included in a matrix. The mosaic image is constituted by a grid (row, column) of such matrixes, which are arranged contiguously both in the horizontal and vertical directions. The characters are either alphanumeric or graphic. FIG. 3 shows an alphanumeric character (in the present case A). Such a character is defined by a shape F, by the character color, i.e. Cc (said color being diagrammatically indicated by sloping stripes) and by the background color, i.e. Cf (diagrammatically indicated by dots). Certain other attributes of the character can be added to the two aforementioned attributes (such as e.g. flashing, height, width, etc.). With regards to the graphic characters, examples will be given hereinafter in connection with FIGS. 9a and 9b.
For certain output equipment only having two colors (e.g. certain printers or flat-faced screens), the background color is necessarily that of the support used (paper in the first case and screen in the second) and the character color is necessarily that of the ink of the tape (for the printer) or that of the excited material (for the screen). If it is a liquid crystal screen, the screen background is generally bright and the character dark. In the case of a cathode ray tube screen, the background is generally dark and the character bright.
These examples show that there is frequently a reversal operation (translated by a binary signal I), making it possible to pass from a display mode to the complementary mode (such as e.g. a bright character on a dark background or a black character on a white background).
The principle of the invention is firstly to establish a correspondence table between the N colors of the input equipment and the M colors of the output equipment. If K0, K1, . . . , KN-2, KN-1 are used for designating the N colors of the input equipment, it is possible to classify the colors in a certain order. As in practice the color information is coded by binary words, this amounts to classifying such words. The left-hand part of FIG. 4 shows the N colors in question in the form of horizontal lines.
For example, for a group of N=8 colors, it is possible to adopt the following classification, which is based on a brightness increase:
______________________________________ N colors N words of n bits ______________________________________ BLACK 000 BLUE 001 RED 010 MAGENTA 011 GREEN 100 CYAN 101 YELLOW 110 WHITE 111 ______________________________________
However, other criteria can be used for classifying the N colors. Moreover, it is advantageous to work with groups of colors containing a number of colors equal to an exact power of 2 N=2.sup.n (in the example considered hereinbefore one has N=2.sup.3). The number of bits of the words translating the colors is then equal to n (to 3 in the above example). However, the invention is obviously not limited to this single case.
It is pointed out that the numerical code chosen is not necessarily the color code used for the display on a color television-type screen, such as screen RT in FIG. 2.
The correspondence table to be established must make it possible to associate with each of the N colors K0, K1, . . . , KN-1, one of the M colors C0, C1, . . . , CM-2, CM-1 of the output equipment. Thus, it is necessary to establish in the same way a second color scale with these M colors. As M is hypothetically less than N, the two scales do not coincide. This second scale is shown in the median part of FIG. 4.
Assuming that the number M is also an exact power of 2, i.e. 2.sup.m, each color C can be associated with a word having m bits, so that m is smaller than n.
Generally the extreme colors C0 and CM-1 are black and white, so that it is logical to make K0 correspond to C0 and KN-1 to CM-1. Transcoding between a color K and a color C only really applies with the intermediate colors.